Apparatuses are well known in the art for separating liquids from slurries. For example, juice presses for separating juice from a fruit are known and examples are shown in U.S. Pat. Nos. 1,971,546; 5,275,097; and 5,365,838. While apparatus for separating juice from fruit was developed in Biblical times, more recently such apparatus has been used in other industries. For example, it is well known in the art that sewage sludge may be dewatered. (In this application, the removal of liquid from a slurry may be referred to a dewatering.) Examples of sewage dewatering apparatus are shown in U.S. Pat. Nos. 5,263,411; 5,307,739; and British 2 119 670 A. A review of these prior patents, as well as many others, shows that problems exist in the art. The product, from which liquid has been extracted, may be referred to as cake. The cake is dropped or otherwise discharged into a bin or other receiving device. If one needs to further extract liquid, the cake must be broken up and conveyed or forced into another apparatus. This is often impossible, or at best difficult.
In single stage batch presses such as the piston presses shown in U.S. Pat. Nos. 5,275,079 and 5,307,729; many products compact so densely that they seal off all exit paths for the escaping liquid. Also, in these designs, it is necessary to clean the filtering media usually at the end of each press cycle. This is done by vibrators, shakers, scrapers, etc. These vibrators, shakers and scrapers are also used to discharge the filter cake from the press as a filter cake frequently sticks badly in batch presses.
A further disadvantage of piston presses such as the type shown in '739 is that it is necessary to provide a valve downstream of the piston to maintain the slurry which is being pressed within the cylinder. As the literature indicates, it is necessary to provide a valve capable of withstanding high pressures and frequently the cost of the downstream valve equals or exceeds the cost of the rest of the apparatus. Continuous presses are also known which apply progressively higher pressures. Examples are shown in U.S. Pat. Nos. 1,971,546; 5,365,838; and GB 2 119 670 A. The auger-type devices shown in '546 and '670 cannot apply a relatively high pressure on the slurry. In addition, auger presses do not aggressively mix the slurry for opening up new passageways for the discharge of material. The '838 reference discloses in FIG. 2 a liquid extracting apparatus which applies progressively higher pressures to a slurry as it is moved away from the inlet on a continuous belt. This design is also defective in that it does not aggressively mix the slurry as it moves from one pressing station to the next for opening up new passageways for the discharge of the material. Also, during the continuous operation, the filter conveyor can clog, and it is not possible to clean the filtering media until the final pressing stage has been accomplished.
It is possible to get more liquid out of a slurry by squeezing it a bit, breaking up the initial cake formed after the initial squeeze, squeezing again, etc. Several presses work on this principle. Two noteworthy examples are the horizontal basket presses of Bucher Guyer in Switzerland, and the pneumatic tank presses of Wilmes in Germany. These presses do the entire dewatering process with the body of the press itself, the problem being the wasted time and effort associated with tying up a relatively large machine during the final stages of pressing, during which time a relatively small quantity of liquid is being expressed.
It is perceived by the present inventors that the product should be removed from the press and transferred to another machine so that the primary presser may be kept employed upon fresh slurry. The secondary presser may be much smaller than the first, as the volume or slurry is greatly reduced when the bulk of liquid is removed in the initial press.